Oil-in-Water Emulsion Polymerization

In emulsifier-free (hereafter referred to as soap-free ) polymerizations, the polymerization is carried out in the same way as described above, except that no surfactant is used. Nucleation occurs by oligoradical precipitation into unstable nuclei which collide to form larger particles. Polymerization takes place mainly within these monomer-swoUen particles, and the particles grow in similar manner to conventional emulsion polymerization. 

Polymers prepared by emulsion polymerization are used either directly in the latex form or after isolation by coagulation or spray-drying of the latex. 

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For practical purposes, styrene—DVB copolymers have commonly been obtained by the suspension polymerization method,[53, 54] which is well known to consist of heating and agitating a solution of initiator in monomers with an excess of water containing a stabilizer of the oil-in-water emulsion. Polymerization proceeds in suspended monomer droplets and, in this way, a beaded copolymer is obtained. While looking very simple, this procedure can provide many complications that significantly change the properties of the beaded product as compared to the properties of materials prepared by bulk copolymerization. AU parameters of the suspension copolymerization have to be strictly controlled, since even small deviations from optimal conditions of the synthesis can serve as an additional source of heterogeneity in the copolymer beads. [55]... 

The imprinted polymer was produced by oil in water emulsion polymerization and, after washing and drying, the imprinted material displayed considerably greater activity than the nonimprinted polymer, as well as the functional host monomer assayed as a catalyst in solution. The preparation of catalytic cavities at the surface of silica particles and other inorganic oxides has also been demonstrated by Markowitz et al. [68,69]. Silica particles were grown in a microemulsion, where the... 

Figure 1 Surface metal imprinting using oil-in-water emulsion polymerization.

Oil-in-water emulsion polymerization systems are typically classified as possessing the characteristics of one of three types of emulsions macro-emulsions, mini-emulsions or microemulsions. These emulsions are the initial systems for emulsion polymerization. There are quite differences between these systems in some aspects such as the size of the droplets (i.e. the discontinuous or dispersed phase), the interfacial area of the droplets, the particle nucleation mechanism and the stability of the emulsion. 

Figure 17.11 SEM image of multihollow polymer particles crosslinked with 50% EGDMA showing the (a) surface and (b) an internal image. (With kind permission from Springer Science + Business Media Colloid Polymer Science, Multihollow polymer microcapsules by water-in-oil-in-water emulsion polymerization Morphological study and entrapment characteristics, 281, 2, 2003, 157-163, J.-W. Kim, J.-Y. Ko, J.-B. Jun et al)...

Kim, J.-W. et al. (2003) MultihoUow polymer microcapsules by water-in-oil-in-water emulsion polymerization morphological study and entrapment characteristics. Colloid and Polymer Science, 281,157-163. 

Inversion emulsion polymerization involves the dispersion and then polymerization of hydrophilic monomers, normally in aqueous solution, in a nonaque-ous continuous phase. The emulsifier systems primarily based on the steric stabilization mechanism (see Section 1.3.3) are quite different from those of the more conventional oil-in-water emulsion polymerization processes. This is simply because the electrostatic stabilization mechanism (see Section 1.3.2) is not effective in stabilizing inverse emulsion polymerization comprising an aqueous disperse phase and a nonaqueous continuous phase with a very low dielectric constant. The unique anionic surfactant bis(2-ethylhexyl) sulfosuc-cinate (trade name Aerosol OT) that can be dissolved in both oil and water... 

The main purpose of pesticide formulation is to manufacture a product that has optimum biological efficiency, is convenient to use, and minimizes environmental impacts. The active ingredients are mixed with solvents, adjuvants (boosters), and fillers as necessary to achieve the desired formulation. The types of formulations include wettable powders, soluble concentrates, emulsion concentrates, oil-in-water emulsions, suspension concentrates, suspoemulsions, water-dispersible granules, dry granules, and controlled release, in which the active ingredient is released into the environment from a polymeric carrier, binder, absorbent, or encapsulant at a slow and effective rate. The formulation steps may generate air emissions, liquid effluents, and solid wastes. 

In a multiphase formulation, such as an oil-in-water emulsion, preservative molecules will distribute themselves in an unstable equilibrium between the bulk aqueous phase and (i) the oil phase by partition, (ii) the surfactant micelles by solubilization, (iii) polymeric suspending agents and other solutes by competitive displacement of water of solvation, (iv) particulate and container surfaces by adsorption and, (v) any microorganisms present. Generally, the overall preservative efficiency can be related to the small proportion of preservative molecules remaining unbound in the bulk aqueous phase, although as this becomes depleted some slow re-equilibration between the components can be anticipated. The loss of neutral molecules into oil and micellar phases may be favoured over ionized species, although considerable variation in distribution is found between different systems. 

Continuous aqueous phase emulsion polymerization is one of the most widely used procedures to make nanoparticles for drug delivery purposes, especially those prepared from the alkylcyanoacrylate monomers. An oil-in-water emulsion system is employed where the monomer is emulsified in a continuous aqueous phase containing soluble initiator and surfactant [39, 40]. Under these conditions, the monomer is partly solubilized in micelles (5-10 nm), emulsified as large... 

The size of the micelles is significantly increased by the addition of monomer up to a diameter of 4.5-5 nm. However, the size of the monomer droplets is stilt very much larger than that of the micelles (diameters up to 1 pm). In emulsion polymerization, one generally uses 0.5-5 wt% of emulsifier relative to monomer. With the usual oil-in-water emulsions, the water content varies from half to four times the amount of monomer. 

It is also possible to generate microcapsules through interfacial polymerization using only one monomer to form the shell. In this class of encapsulations, polymerization must be performed with a surface-active catalyst, a temperature increase, or some other surface chemistry. Herbert Scher of Zeneca Ag Products (formerly Stauffer Chemical Company) developed an excellent example of the latter class of shell formation (Scher 1981 Scher et al. 1998). He used monomers featuring isocyanate groups, like poly(methylene)-poly(phenylisocyanate) (PMPPI), where the isocyanate reacts with water to reveal a free primary amine. Dissolved in the oil-dispersed phase of an oil-in-water emulsion, this monomer contacts water only at the phase boundary. The primary amine can then react with isocyanates to form a polyurea shell. Scher used this technique to encapsulate pesticides, which in their free state would be too volatile or toxic, and to control the rate of pesticide release. 

Following route A (Fig. 1), Yan Xiao et al. reported the chemoenzymatic synthesis of poly(8-caprolactone) (PCL) and chiral poly(4-methyl-8-caprolactone) (PMCL) microparticles [5]. The telechelic polymer diol precursors were obtained by enzymatic polymerization of the corresponding monomers in the presence of hexanediol. Enzymatic kinetic resolution polymerization directly yielded the (R)-and (S )-enriched chiral polymers. After acrylation using acryloylchloride, the chiral and nonchiral particles were obtained by crosslinking in an oil-in-water emulsion photopolymerization. Preliminary degradation experiments showed that the stereoselectivity of CALB is retained in the degradation of the chiral microparticles (Fig. 2). 

The rubber particle size in the final product increases several fold if the prepolymerization is carried out in the presence of a dilute aqueous solution of an alkane sulfonate or polyvinyl alcohol in place of pure water. The addition of a surface-active agent converts the coarsely dispersed oil-water mixture—obtained as above in the presence of pure water—into an oil-in-water emulsion. In this case even prolonged stirring during prepolymerization does not decrease the rubber particle size appreciably in the final product. The stabilization of the droplets of the organic phase in water by the emulsifier obviously impedes or prevents agitation within the polymeric phase. Figure 1 shows the influence of these three prepolymerization methods (under otherwise equal reaction conditions) on the dispersion of rubber particles in polystyrene. 

FIGURE 13.19 Drug loading of polymeric micelles by the dialysis (a) and the oil-in-water emulsion methods (b). (Reproduced from Jones, M. C. and J. C. Leroux. 1S90. J. Pharm. Biopharm48 101-111. With... 

The emulsifying properties of these polymeric surfactants demonstrate that the chemical structure influences the kinetic behaviour of interfacial tension reduction. An increase of sulfopropyl moieties reduces the interfacial tension slower while an increase in 2-hydroxy-3-phenoxy propyl moieties reduces the interfacial tension faster. The ionic strength of the emulsion appears to increase the rate of tension reduction. The average droplet size of oil-in-water emulsions in presence of previously dissolved 2-hydroxy-3-phenoxy propyl sulfopropyl dextran is around 180 nm immediately after preparation and increases with time. The presence of ionic moieties appeared to facilitate emulsification at low polymer concentrations due to electrostatic repulsions between the oil droplets [229]. 

Figure 9. Hydrophilic polymeric surfactant at the interface in an oil-in-water emulsion.

Cosmetic suspensions are available in two types. The first comprises pigmented products that are suspended in essentially aqueous vehicles (liquid makeup, eyeliners, mascara, and blusher). These products have a high solids content, high density, impalpable powders, and pigments permanently suspended in a primary oil-in-water emulsion-type base or a complex system of hydrophilic cellulose derivatives, clays, and/or polymeric film formers, in which the gelling and suspending properties of the vehicle often are reinforced by a small amount of a Bingham-type plastic such as carbomer. 

Another type of emulsion-like process is called miniemulsion polymerization. Miniemulsions are stable oil in water emulsions with average droplet diameter of 80-400 nm, prepared using a mixture of an anionic emulsifier and a cosurfactant such as a long-chain fatty alcohol or n-alkane. The polymer latexes are prepared by initiation of polymerization in the miniemulsion droplets. 

In bulk- and solution-phase free-radical polymerization, there is a tradeoff between molecular weight and polymerization rate. This is especially true for controlled/living radical polymerization. In emulsion polymerization, however, high molecular weight polymers can be made at fast polymerization rates. Emulsion polymerization is a type of radical polymerization that is frequently used for making polymers of high molecular weight. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified with surfactants in a continuous phase of water. 

Figure 14.13 Oil-in-water emulsions may be stabilized by (A) non-ionic surfactants, [B) poloxamer block copolymers or [C) polymeric materials. The hydrophilic chains produce repulsion by mixing interaction [osmotic) or volume restriction [entropic).

Bartelt [5]. Therefore, due to historic precedent the description of water-in-oil polymerizations proceeds through an analogy, either colloidal or kinetic, to a process with a continuous aqueous phase. The prefix inverse is generally accepted for water-in-oil emulsions in contrast to direct or conventional oil-in-water emulsions/microemulsions for which the prefix is implied but not often explicitly stated. 

The first step in all interfacial polymerization processes for encapsulation is to form an emulsion. This is followed by initiation of a polymerization process to form the capsule wall. Most commercial products based on interfacial or in situ polymerization employ water-immiscible liquids. For encapsulation of a water-immiscible oil, an oil-in-water emulsion is first formed. Four processes are schematically illustrated in Figure 5.82. In Figure 5.82(a), reactants in two immiscible phases react at the interface forming the polymer capsule wall. For example, to encapsulate a water-immiscible solvent, multifunctional acid chlorides or isocyanates are dissolved in the solvent and the solution is dispersed in water with the aid of a polymeric emulsifier, e.g., poly(vinyl alcohol). When a polyfunctional water-soluble amine is then added with stirring to the aqueous phase, it diffuses to the solvent-water interfece where it reacts with acid chlorides or isocyanates forming the insoluble polymer capsule wall. Normally some reactants with more than two functional groups are used to minimize a regation due to the formation of sticky walls. 

In the encapsulation process depicted in Figure 5.82(c), polymerization is initiated in the water phase of oil-in-water emulsion. As the molecular size of the polymer increases, it deposits at the water-oil interface where it continues to grow forming a cross-linked polymer capsule wall. In a typical example, methylol urea or methylolmelamine is added to the aqueous phase along with an ionic polymer. The pH is adjusted to 3.5—4.5 and the mixture allowed to react for 1-3 h at 50-60°C. The ionic polymer in the aqueous phase assists deposition of the aminoplastic at the water-oil interface. 

Monofunctional hydroxyl terminated polyethylene oxide chains with degree of polymerization from 5 to 20 are reacted with pMDI, or simply with MDI, to provide surface active agents. The resulting surface active agent is then mixed with pMDI to provide a resin which is dispersible in water, resulting in an oil-in-water emulsion [51]. Such emulsions are stable for brief periods, 1 to 2 hours, before the water reaetion eauses gelation. Emulsifiable pMDI could be used where dispersion in water offers some benefit. For example, neat emulsifiable pMDI could be added directly to the blow line for medium density fiberboard production. Water emulsified pMDI has been used for improving resin distribution in particleboard or OSB manufacture however, this is not common industrial practice. 

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